Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. The base stocks typically include mineral oils, highly refined mineral oils, poly alpha olefins (PAO), polyalkylene glycols (PAG), phosphate esters, silicone oils, diesters and polyol esters. Polyol esters have been commonly used as base stocks in lubricant application where thermal and oxidative stability are critical. Despite their inherent thermal/oxidative stability as compared with other base stocks (e.g., mineral oils, polyalpha olefins, etc.), even these synthetic ester lubricants are subject to oxidative degradation and cannot be used, without further modification, for long periods of time under oxidizing conditions. It is known that this degradation is related to oxidation and hydrolysis of the ester base stock.
Conventional synthetic polyol ester lubricant oil formulations require the addition of antioxidants (also known as oxidation inhibitors). Antioxidants reduce the tendency of the ester base stock to deteriorate in service in which deterioration can be evidenced by the products of oxidation such as sludge and varnish-like deposits on the metal surfaces, and by viscosity and acidity growth. Such antioxidants include arylamines (e.g., dioctyldiphenylamine (V-81) and phenylalphanaphthylamine), and the like.
Frequently replacing the lubricant oil or adding an antioxidant thereto to suppress oxidation increases the total cost of maintaining the engine or machine. It would be most desirable to either enhance the efficiency of conventional antioxidants (i.e., provide increased antioxidant potency in formulated products) or produce an ester base stock which exhibits substantially enhanced thermal/oxidative stability compared to conventional synthetic ester base stocks, and wherein the ester base stock does not require frequent replacement due to decomposition (i.e., oxidation degradation). It would also be economically desirable to eliminate or reduce the amount of antioxidant which is normally added to such lubricant base stocks.
Upon thermal oxidative stress a weak carbon hydrogen bond cleaved ultimately resulting in an unstable alkylperoxyl radical on the ester. The role of conventional antioxidants is to transfer a hydrogen atom to the unstable carbon radical and effect a “healing” of the radical. The following equation demonstrates the effect of antioxidants (AH):AH+ROO·→A·+ROOH
The antioxidant molecule is converted into a radical, but this radical (A·) is far more stable than that of the ester-based system. Thus, the effective lifetime of the ester is extended. When the added antioxidant is consumed, the ester radicals are not healed and oxidative degradation of the polyol ester composition occurs. One measure of relative thermal/oxidative stability well known in the art is the use of high pressure differential scanning calorimetry (HPDSC).
HPDSC has been used to evaluated the thermal/oxidative stabilities of formulated automotive lubricating oils (see J. A. Walker, W. Tsang, SAE 801383), for synthetic lubricating oils (see M. Wakakura, T. Sato, Journal of Japanese Petroleum Institute, 24 (6), pp. 383-392 (1981)) and for polyol ester derived lubricating oils (see A. Zeeman, Thermochim, Acta, 80(1984)1). In these evaluations, the time for the bulk oil to oxidize was measured which is the induction time. Longer induction times have been shown to correspond to oils having higher concentrations of antioxidants or correspond to oils having more effective antioxidants or at a fixed level of a given antioxidant, have been shown to correspond to oils having intrinsically more stable base stocks. For automotive lubricants, higher induction times have been correlated with viscosity break point times.
The use of HPDSC as described herein provides a measure of stability through oxidative induction times. A polyol ester can be blended with a constant amount of dioctyl diphenylamine which is an antioxidant. This fixed amount of antioxidant provides a constant level of protection for the polyol ester base stock against bulk oxidation. Thus, oils tested in this manner with longer induction times have greater resistance to oxidation. For the high hydroxyl esters of the present invention in which no antioxidant has been added, the longer induction times reflect the greater stability of the polyol ester high hydroxyl molecules by and also the natural intrinsic antioxidancy of the esters due to the unesterified —CH2OH groups.
The present inventors have developed a unique polyol ester composition having enhanced thermal/oxidative stability when compared to conventional synthetic polyol ester compositions. This was accomplished by synthesizing a polyol ester composition from a polyol and branched and/or linear carboxylic or aromatic acid in such a way that it has a substantial amount of unconverted —CH2OH groups. Having a highly branched polyol ester backbone enhances the ability of the high hydroxyl ester to act similarly to an antioxidant, i.e., cause the thermal/oxidative stability of the novel polyol ester composition to drastically increase, as measured by high pressure differential scanning calonmeizy (HPDSC). This selectively branched polyol ester composition restricts the mechanisms by which the non-CH2OH portion of the high hydroxyl ester oxidizes and thereby provides an intramolecular mechanism which is capable of scavenging alkoxide and alkyl peroxide radicals, thereby substantially reducing the rate at which oxidative degradation can occur.
The thermal and oxidative stability which is designed into the novel polyol ester compositions of the present invention eliminates or reduces the level of antioxidant which must be added to a particular lubricant, thereby providing a substantial cost savings to lubricant manufacturers. Furthermore, the novel high hydroxyl polyol esters of the present invention are capable of decreasing the level of other antioxidants required when they are grafted to other formulated products including lubricants, fuels, oligomers and polymers, thus potentially reducing the cost of formulation to achieve desired performance targets.
Another problem of conventional formulations is that the ability to dissolve conventional antioxidants in the respective base stock varies widely and in some instances the use of the selected antioxidants is prohibited due to solubility limitations. The present invention also offers a means of incorporating additives into formulations that may be poorly soluble by attaching them through conventional hydrocarbon bonds to the additive in question. The high hydroxyl ester thus provides not only solubility but provides antioxidancy to the additive in question.
The high hydroxyl polyol esters of the present invention, when grafted onto oligomers, polymers or the like are capable of providing the enhancement of oxidation stability without the potential debits associated with conventional antioxidants that are known to produce color bodies, a significant debit for selected polymer applications.
The present invention also provides many additional advantages which shall become apparent as described below.